Revertive call circuit



Oct. 2, 1956 H. L. HoRwn-z Er AL 2,765,368

REVERTIVE CALL CIRCUIT 76 IRP 75 ZKF 75 /P fo ZEP INVENTOR. Hl..HO/PW/TZ BY G l.. 'xL/A555@ Oct- 2, 1956 H. L HoRwlTz Er AL 2,765,368

REVERTIVE CALL CIRCUIT 2 Sheets-Sheet 2 Filed Nov. 12, .1953

INVENTOR. H HRW/TZ 6.6.. HSSE 46E/vr United States Patent O REVERTIVECALL CIRCUIT Henry Leopold Horwitz, Nutley, and George Louis Hassel',Packanack Lake, N. J., assignors to International Telephone andTelegraph Corporation, New York, N. Y., a corporation of MarylandApplication November 12, 1953, Serial No. 391,524

Claims. (Cl. 179-17) This invention relates to a reverting call ringingcircuit for automatic telephone switching systems.

In automatic telephone systems in which station lines are equipped formulti-station or party line Working, certain of the party lines may bearranged for bridged ringing, whereas others may preferably be arrangedfor divided ringing.

Where the number of stations connected to a party line does not exceedfour or live, it is common practice to provide at the exchange sourcesof alternating ringing current of four or ive different frequencies andto equip each station with a ringer connected in bridge across the lineconductors and tuned to respond to ringing current of one only of thesefrequencies, whereby the selection of a particular station on a partyline is effected by selecting the frequency of the ringing current to beapplied thereto. rl`hus, when a call is made from another line to astation on a multi-party line, the ringer at that station alone respondsto the ringing current and no other party on the line is disturbed.

When it is desired to increase still further the number of stationsconnected to a party line, up to a maximum of eight or ten stations, itis usual, rather than to increase the number of ringing current sourcesat the exchange, to divide the stations into two groups, each station ofa group having a ringer tuned to a different frequency, and to connectthe ringers to the line according to one of two methods, depending uponthe physical characteristics of the line.

According to the preferred method, applicable to lines having goodinsulation quality and not subject to interference from neighboringpower or other lines, the ringers at the stations of one group areconnected between one of the line conductors and ground and those of theother group between the other line conductor and ground, whereby theselection of a particular station on the line is secured by selectingboth the frequency of the ringing current to be supplied to the line andthe particular conductor thereof to which it shall be applied. In thiscase also, the ringer at only one station on a party line responds tothe ringing current when a call is made to that station from anotherline.

According to the second method, applicable to lines having lessfavorable insulation quality and/ or subject to interference ifunbalanced with respect to ground, it is usual to connect the ringers atthe stations of both groups in bridge across the line conductors and todistinguish between two stations, one in each group, having ringerstuned to the same frequency by assigning a different ringing code toeach, for example, a one-ring code to the stations of one group and atwo-ring code to those of the other group. This second method is subjectto the obvious objection that two stations are rung when a call is madeto either of them from another line, the party at one of the stationsbeing unwanted and thus unnecessarily disturbed; for this reason, such aparty line is often described as a semi-selective party line.

When a reverting call is made from a station on a 2,765,368 PatentedOct. 2, 1956 ICC multi-party line, that is, a call to another stationconnected to the same party line, it is common practice to require thecalling party to replace his handset on the cradle after the completionof dialing, whereupon the exchange proceeds to apply the appropriatekind of ringing current to the party line to ring the called station,the ringing being tripped in the usual way when the called party answersthe call. In order that the calling party may know when the answeroccurs, and thus when he may himself lift the handset again and beginconversation, the exchange is arranged to apply two kinds of ringingcurrent to the party line alternately, one kind to ring the calledstation, as stated, and one kind to ring the calling station; thecalling party then lifts the handset when his own ringer ceases torespond.

However, when a semi-selective party line is involved, that is, onearranged for bridged ringing together with a choice of ringing codes inaccordance with the second method described hereabove, a ditiicultyarises when a reverting call is made to the one other station on theline v having a ringer tuned to respond to the same frequency as theringer at ,the calling station. For, in that case, both ringers wouldrespond to both ringing codes alternately and the called party mightwell fail to recognize, and therefore might disregard, his own ringingcode. Accordingly, it is known practice to suppress the ringing code ofthe calling station when that station will in any case be rung by theringing current directed at the called station.

It will be evident that, when the party line involved is one which isarranged for divided ringing in accordance with the first methoddescribed hereabove, the exchange must in no circumstances suppress theringing of the calling station, even though the ringing frequenciesassigned to both calling and called stations be the same. It followsthat, unless the exchange, in order to handle reverting calls ou partylines of both kinds, comprises means for determining whether the line-involved in a particular reverting call is arranged for bridged or fordivided ringing, the party lines in the exchange having more stationsconnected thereto than there are ringing current frequencies availableare preferably all arranged either for bridged ringing or for dividedringing, an undesirable restriction of the flexibility of the exchange.

It is an object of our invention to provide a reverting call ringingcircuit which may be taken into use when a call is made from a stationon a party line to another station on the same line and which isarranged to select the proper ringing current frequencies for ringingboth the calling and called stations, to determine whether the partyline concerned is arranged for divided or for bridged ringing, and toapply the appropriate ringing currents to the line alternately in theproper manner.

A further object of our invention is to suppress the ringing code of thecalling station from which a reverting call is being made if the ringingcurrent frequency selected for ringing the calling station is the sameas that selected for the called station and the party line concerned isarranged for bridged ringing.

A still further object of our invention is the provision of means forforced release of a reverting call ringing circuit if the call isneither answered by the called party nor cancelled by the calling partywithin a predetermined time-interval.

According to one feature of our invention, therefore, a reverting callringing circuit is provided with means for determining whether a partyline on which a reverting call is being made is arranged for divided orbridged ringing, and means responsive to impulses dialed by the callingparty for applying ringing currents of the proper frequencies and to theproper side or sides of the line to ring the calling and called stationsthereon alternately in the case that the'party line concerned isarranged for dividedY ringing, and for applying to the line ringingcurrent of the proper frequencies and interrupted at appropriateintervals to ring the calling and called stations thereon alternately inthe case that the party line concerned is arrangedfor bridged ringing.

According to another feature of the invention, the dialed impulsesconstituting the party digits of the calling and called stations arecaused to position respectively a pair of step-by-step switches in thereverting call ringing circuit, which switches select the frequencies ofthe ringing currents to be applied to the party line and determine theside or sides of the line to which they are to be applied in case theline isV arranged for divided ringing, or the codes of the ringingcurrents in case the line is arranged for bridged ringing.

Accordingv to yet another feature of the invention, the switches in theVreverting call circuit also cause the ringing current normally appliedto Vthe party line to ring the calling party to bel suppressed if thefrequency of the ringing current selected by the party digit of thecalled party is the same as that selected by the party digit of thecalling party, provided the party line is one which is arranged forbridged ringing, but not if it is arranged for divided ringing.

The above-mentioned and other objects and featuresrof our invention willbe clearly understood from a consideration of the following descriptionof a preferred embodiment thereof, taken in conjunction withtheaccompanying drawings, in which:

Fig. 1 shows parts of the circuit of a multi-party line and blockdiagrams of a line finder and a group selector, respectively. o Y

Fig. 2 is a detailed showing' of a reverting call ringing circuitaccordingv Vt'o'our invention,'and should be placed to the right of Fig.1 `for a proper understanding thereof.

Figs. 3, 4 and 5 show modifications of the circuit of Fig. 2.

In the drawings, the convention universally adopted for indicating relaycontacts detached from their respective relay coils has 4been employedfor simplifying vthe tracing of the circuits. VIn this form of showing,a relay such as H, Fig. 2, Vis given a letter designation placed above anumeral, thus:

The numeral indicates the number of contact combinations associated withthe relay H, each of these combinations beingV given a designationconsisting of thev same letter followed by a numeral, thus: H1, H2, H3and H4. The contacts associated with any particular relay can then bereadily located on the drawings. A

With reference Vto Fig. 1, a party line having a plurality of stationsconnected thereto, of which two only,.T1 and T2, are shown, is providedat the exchange with a lineV circuit of conventional lock-out type,Vhaving a line relay L, a cut-off relay CO and a lock-out relay LO,which is connected to a set of terminal contacts in the bank of each ofa group of line iinders, of which one only is shown. The line findersmay be of any convenient construction suitable for extending callinglines connected to their banks to group selectors. In the preferredembodiment of Vour invention, the line finder is fa conventionalstep-by-step nder and is accordingly shown in block form and will not bedescribed in detail. Similarly, the group selector is a conventional oneor twodigit dial-operated step-by-step selector of the two-motion typeYand is also shown in block form. It has a plurality of revertive callringing circuits connected to sets of terminal contacts in its bank, ofwhich one'is shown in detail in Fig. 2, connected by trunk conductorsand and controliconductor P. ',v

When a subscriberV at a party line station, for example, station T1,Fig. 1, lifts his handset from the cradle to originate a call, the linerelay L operates and causes a Y line finder, such asl the one shown, toplace its wipers on the contacts connected'to the party line whereafterthe cut-ofi relay CO operates to ground placed on conductor Pi, relay Lrestores, and the line conductors are extended to the group selectorshown, all in a manner well known in the art and therefore' notdescribed in detail. The line circuit being provided, with lock-outfacilities, the lock-out relay LAO also operates. In order to place areverting call Vto another station, Vfor example station Thon the sameparty line,` the calling party dials the one or two digitsassignedftothisservice and the selector proceeds to select a revertivecall ringing circuit, such as that shown in Eig. 2, alsoY in a wellVknown manner, and vextends the party line conductors to the trunkconductors andY |-"i, and conductor P1 to the control conductor P.

With reference now to Fig. 2, which will be described in greater detail,the revertive call ringing circuit comprises, in addition to a number ofrelays, a pair of stepby-step switches, MSA and MSB, of the well knownminor switch type, each having three wipers and corresponding rows ofbank contacts, MSAl, 2, 3 and MSBI,

Y 2, 3, an operating magnet, OMA and OMB, a release magnet, ZMA and ZMB,and olf-normal contact springs, CMA1, 2 and CMBl, 2, respectively. Thecontacts of banks MSA!L and MSBI are each connected to one of tiveringing current generators RGl to RGS, which may be common to aplurality of ringing circuits andl generate alternating currents at livediierent frequencies; contacts 1 and 6 of both banks are connected togenerator RG 1, contacts 2 and 7 vto GRZ, etc., for a purpose to beexplained. Y i

When the reverting call ringing circuit is seized, as described, thecircuit to relay A is closed via the loop at the calling station. Aoperates and closes the circuit to relay B at make contact A1. BYoperates, prepares part of the irnpulsing circuit to the operatingmagnets OMA and OMB at make contact B1, prepares part of circuits torelaysv F, G and H and closes the circuit to the operating windings ofrelays E and C at make contact B3, and places ground on the controlconductoriP at makecontact B2 to make the reverting call circuit busy toother testing selectors and also to hold the preceding switchesoperated. Relay C operates and prepares the impulsing circuit to itsholding winding and magnet OMA in parallel at make Contact C1; contactC2 is not effective at this time. Relay E operates'also and furtherprepares the impulsingV circuit at make contact E2; contacts E1 and E3are not effective at this time.

The calling party now dials the party digit assigned to his own station,followed by the party digit Yassigned to the called station. Thosestations on each party line having fingers tuned tothe frequency ofgenerator RGI are assigned the party digit 1 or 6, those whose ringersrespond to generator RGZ have the digit 2 or 7, etc. Y The distinctionbetween the stations assigned the party digits 1 to 5 and those assignedthe digits 6 to 0 will be eX- plained subsequently. Y

Relay A follows the first train of impulsesV received from the dial andeach time it restores it opens the circuit to relay B and at breakcontact lA1 closesthe impulsing circuit to the holding winding of relayC and to magnet OMA, via the previously closed make contacts B1, E2 andC1. OMA follows the impulses received from A and causes the wipers ofswitch MSA to rotate step-bystep to the bank contacts corresponding tothe digit dialed, thus selecting the calling stations ringing frequencyover wiper MSAl and preparing part of the circuit for applying ringingcurrent to the calling line. At the rst step of switchr MSA, theoff-normal springs CMA1 and CMAZ operate and open the circuit to theoperating winding of `C and closepart of the circuit to the releasemagnet ZMA, respectively. Relays VKB and' C are slow to release, owingto the 'presence of copper sleeves on their 'coil cores and accordinglyremain operateddun'ng the trainof impulses.

After the last impulse ofv the train, relay A remains operated,re-closing the circuit to relay B and opening the circuit to the holdingwinding of C and to OMA. Relay C on restoring transfers the impulsingcircuit to the holding winding of relay E and magnet OMB in parallel atcontacts C1, and prepares part of the circuit for applying ringingcurrent to the calling line at break contact C2.

Relay A follows the second train of impulses received and each time itrestores it opens the circuit to relay B and closes the circuit to theholding winding of relay E and to magnet OMB, via break contact C1. OMBfollows the impulses received from A and causes the wipers of switch MSBto rotate to the contacts corresponding to the digit dialed, thusselecting the called stations `ringing frequency over wiper MSBI andpreparing another part of the circuit for applying ringing current tothe line. At the first step of switch MSB, the ofi-normal springs CMBland CMB2 operate and open the circuit to the operating winding of E andclose part of the circuit to the release magnet ZMB, respectively. RerayE is slow toV release, owing to the presence of a copper sleeve on itscoil core and remains operated, together with relay B, during the trainof impulses.

After the last impulse of this train, relay A remains operated,re-closing the circuit to relay B and opening the circuit to the holdingwinding of E and to OMB. Relay E on restoring opens the impulsingcircuit and prepares part of the circuit to relay M at transfer con.tacts E2.

Upon completion of dialing, the calling party replaces his handset onthe cradle, thus opening the circuit to relay A. A restores, opens thecircuit to relay B and closes the circuit to relay M via break contactE2. Relay M is slow to operate owing to the presence of a copper slug atthe armature end of its coil core, to prevent premature operation ofrelay F. M on operating provides another circuit at make contact M1,under the control of relay E and a ringing timer, to hold relay B,disconnects one winding of relay A from the trunk conductor of breakcontact M2, connects the interruptercontact PU to the timing relays Gand H at make con` tact M3, opens the circuit to contacts E2 and closesa locking circuit to itself at make-before-break contacts M4,disconnects the other winding of relay A from the trunk conductor atbreak Contact M5, prepares parts of the ringing circuit to the and trunkconductors at make contacts M2 and M5, prepares circuits to relays L andD at make contact M7, connects ground to conductor RST in the ringinginterrupters at make contact M8, and prepares a circuit for the re-operation of relay E from the ringing timer at make contact M6.

The ringing interrupters, which may be common to a plurality ofreverting call circuits, have the purpose of producing ground pulses tocause the ringing current projected over party lines to be interruptedcyclically in accordance with .a one-ring or a two-ring code, as:required, and of ensuring that ringing starts only at the beginning of acode cycle. They may be of any suitable construction and are shown inthe form of three cam operated contacts PU, lRP and ZRP, providingrespec tively, in each cycle, preferably of three seconds, a short'.pick-up pulse, a single pulse for the one-ring code and two pulses forthe two-ring code; the cams are mounted on the same shaft, driven by amotor, not shown, which may be started by applying ground to theconductor RST..

Accordingly, ground from the pick-up contact PU at the beginning of thenext ringing cycle closes the circuit to one winding of relay H and toboth windings of relay G. G does not operate at this time becausethemagnetic fields associated with its two windings oppose each other.Relay H operates, however, closes part of the ringing circuit forringing the called station via the MSBI bank contact at make contact H2,closes an alternative circuit to its operating Winding and to onewinding of relay G at make-before-break contacts H4, prepares part ofthe circuit to relay L via the MSB2 bank contact and closes the circuitto relay D at make contact H3, and disconnects one winding of relay Kfrom the trunk conductor at break contact H1. Relay D operates, closesits own locking circuit at make contact D1, prepares a circuit to relay.l at make contact D4, connects relay L to certain of the MSA2 and MSB2bank contacts at make contact D2, and connects ground to conductor TSTat make contact D3.

Relay L operates if switch MSB has been stepped to a position whereground via contacts M7 yand H3 is connected to the MSBZ bank contact,and at make contact L3 prepares part of the circuit for applying ringingcurrent to the i trunk conductor. If relay L does not operate, at itsbreak contact L2 it prepares part vof the circuit for applying ringingcurrent to the trunk conductor. Also, if relay L operates, the circuitto relay J is closed to the two-ring pulse interrupter 2RP via ContactD4, whereas if L does not operate, the circuit to J is closed to theone-ring pulse interrupter lRP. The contacts of banks MSA2 and MSB2which are wired to contact D2 are therefore those which correspond tothe party digits of the stations which have their ringers connectedbetween the line conductor and ground in the case of a line arranged fordivided ringing, and of the stations to which a two-ring code isassigned in the case of a line arranged for bridged ringing. In theembodiment shown in Fig. 2, each of these stations has one of the partydigits 6, 7, 8, 9 or O.

When ground is removed by contact PU, the circuit to one winding ofrelay G is opened, allowing this relay to operate over its otherwinding. Relay G connects the second winding of relay H to contact PU atmake contact G1 and connects one winding of relay K to one of the trunkconductors at make contact G2, via make contact H1 and either breakcontact L3 and make contact M5 or make contacts L2 and M2.

The next ground pulse from the interrupter contact IRP or 2RP, as thecase may be, closes the circuit to relay I. l operates, closes theringing circuit at make contact Il, and disconnects resistor YD at breakcontact J2. v

lf bridged ringing is utilized on the party line connected at this timeto the reverting call ringing circuit, ringing current at the frequencyassigned to the called station ows from one of the battery-connectedgenerators RG1 RG5, via a bank Contact MSB, currentlimiting resistor YB,make contacts H2 and I1, one winding of relay F, break contact L2 ormake contact L3, make contact M2 or M5, the or trunk conductor, theringer and condenser at the called station, the i or trunk conductor,make contact M5 or M2, break contact L3 or make contact L2, makecontacts G2 and H1, rectifier MRA and one Winding of relay K or reotierMRB, to ground at make contact B3. Relay K operates on that portion ofthe ringing current which passes through rectifier MRA, locks at makecontact K1 via its second winding, short circuits its operating Windingand the two rectiliers at make contact K3 to improve the ringingconditions, and prepares a circuit to re'operate relay C at make contactK2, for a purpose to be described hereafter. The ringer at the calledstation responds to the ringing current; if there is another station onthe line at which the ringer is tuned to the same frequency, that ringerresponds also. A copper slug on the coil core of relay F lowers itsimpedance to prevent the operation of this relay on the ringing current.

If divided ringing is utilized on the party line concerned, ringingcurrent flows in the circuit traced in the preceding paragraph to theringer and condenser at the called station and returns yto the exchangevia the ground 7 connection at that station. The Yringer at the calledstation alone responds and relay K, which is not included in the ringingcircuit, cannot operate.

The operation or non=operation of relay K during the rst ringing periodis thus an indication thatV the party line concerned is arranged forbridged or divided ringing,

respectively. Y

Relay l follows the pulses received from the interruptor contact IRP orZRP and applies a one-ring code r a two-ring code to the line accordingas relay L is unoperated or operated, thus enabling the party at cach ofthe two stations where the ringer responds, in the case of bridgedringing, to determine Whether or not he is being called; in the case ofdivided ringing, the ringing code has no significance. During eachsilent period, when relay l is unoperated, the connection to the gewerator is replaced at transfer contacts I1 by a connection to batteryvia current limiting resistor YC, to permit tripping of the ringingduring this period also; and at break contact J 2 resistor YD is shuntedacross the winding of relay K and its associated rectifiers MRA and MRB,to absorb condenser discharges from the line when relay .l restores.

After one complete cycle of the ringing interrupters, that is, whencontacts lRP and 2RP are both open, contactPU closes again, thus closingthe circuit to the second winding of relay H, via make contact G1. H nowrestores, because the magnetic fields associated with its two windingsoppose each other, transfer to contact PU the Vcircuit to its operatingwinding and to one winding of relay G at Vcontacts H4, transfers theringing circuit from wiper MSBl to wiper MSAl at contacts H2, transfersthe circuit to relay L from wiper MSBZ to wiper MSA2 at contacts H3, andagain disconnects one winding of ,relay K from the trunk at make contactH1.

Relay L now takes up a position, operated or unoperated, according tothe setting of switch MSA, that is, according as the dialed party digitof the calling party was between 6 and 0 or between l and 5,respectively, preparing the ringing circuit to ring the calling party atcontacts L2 and L3 and selecting the appropriate ringing code atcontacts L1. Relay L is slow to release, by

virtue of the resistor YE Vconnected in parallel with its,

MRA and MRB to one of the trunk conductors at break r contact G2, thistime Via break contact H1.

The next ground pulse from contact lRP, or two pulses from contact 2R17,as the case may be, operate and release relay I; and Vthis relay, exceptin one particular case to be described hereafter, closes the ringingcircuit, this time from a bank contact MSAl, via current-limitingresistor YA, break contacts C2 and H2, andthence as previouslydescribed, and accordingly projects over the party line one or twobursts of ringing current at the frequency assigned to the callingparty. The ringer at the calling station responds; if there is anotherstation on the line at which the yringer is tuned to the same frequency,that ringer responds also.

After another complete cycle of -the ringing interrupters, contact PUcloses, re-operating relay H, and then opens, allowing relay G torte-operate, as before, after which the appropriate ringing current toring the called party is again projected over the line, in the manneralready explained. During the next cycle, with relays H and G bothunoperated, the calling station is again rung, whereafter the operationsdescribed in this paragraph are repeated.

of no reply, removes his own handset frointhe. cradle to trip theringing and then disconnects to; cancel the call. If the calling partyvabandons the call and ringing conv tinues, the connection is in duecourse forcibly Vreleased by the ringing timer, as will be explained.

The purpose of ringing the calling station as well as the called stationis to inform the calling party, after he has replaced his handset on thecradle, that the called station is infact being rung and to indicate, bythe cessation of ringing, the moment when he may again lift the handsetand start conversation. However, if it so happens that both stations areconnectedY to a party line arranged for bridged ringing and the ringersthereat are both tuned to the same frequency, then the ringer at thecalling station will respond to theringing current directed at thecalled station, and Vice-versa, as stated above;

ln such a case, it is clearly unnecessary, for the abovementionedpurpose, to project ringing current interrupted at the calling partyscode at all; indeed, to do so would tend to confuse the called party,who would hear his own code and that of the calling party alternatelyand might well disregard the call altogether. In order to suppress theringing current directed at the calling station in this particular case,the ,switches MSA and MSB Vare each provided with another wiper, MSA3and MSB3,

respectively; the bank contacts 1 and 6, 2 and 7, etc., of one switchare wired together and tothe corresponding bank contacts of the otherswitch, and the combination is included in the aforo-mentioned circuitto re-operate relay C which is prepared by the closure of make contactK2.

In the particular case, therefore, where wipers MSAI and MSBl have bothbeen positioned tov select the same ringing generator, that is,respectively, on bank contacts 1 Vand 6, 2 and 7, etc., or vice-versa,and provided relay K has operated during the iirst ringing of the calledstation, the circuit to relay C is closed, via contact K2 and bankcontacts MSB3 and MSA3, to ground at contact B3. Relay C operates andopens the circuit for ringing the calling station at break contact C2.Thus, both stations. are rung repeatedly by the ringing current directedat the called party and that directed at the calling party issuppressed, but only if the party line concerned is arranged for bridgedringing. It is accordingly possible to arrange any individualmulti-party line either for fully selective divided ringing or forsemi-selective bridged ringing with one-ring and two-ring codes, ascircumstances may dictate, and the flexibility of the system is therebyenhanced.

It will have been noted that, with the circuit arranged in accordancewith Fig. 2, two-ring or one-ring coded ringing is used, according asrelay L does or does not operate, even when the party line concerned isarranged for divided ringing and a single code would suice. Fig. 3 showsa modification of the circuit whereby the onering code is usedexclusively in connection with such a line. Additional transfer contactsK4 are inserted in the operating circuit to relay J; ir relay K remainsunoperated,

indicating divided ringing, relay I is connected to contact ERPindependently of contacts L1, whereas if relay K operates, indicatingbridged ringing, the circuit of relay J is thereafter controlled bycontacts L1 as in Fig. 2. To

ensure that relay J does not restore during the transfer of contacts K4,a suitable resistor YF is connected in parallel with the winding of land contact ZRP is adjusted to be closed at the moment relay K operates.

Fig. 4 shows an alternative modification of the circuit of Fig. 2,whereby the two-ring code is used exclusively when the party lineconcerned is arranged forV divided ringing. The break contact K4 isconnected to contact ZRP; otherwise the modication is the same as inFig'.`3.

With reference again to Fig. 2, it has been stated that when the callingparty replaces his handset on the cradle; to intitiate theringingoperation, relay M operates, relay A restores and the circuit torelay Bis placedunder the control of relay E and a ringing timer. The latter,which may be common to a plurality of reverting call ringing circuits,has the purpose of Eorcing the release of the circuit connected to it ifthe ringing from that circuit is not tripped within a predetermined timeinterval. The timer may be of any suitable construction and is shown inthe form of two cam-operated contacts, of which one, T1, opens for ashort interval during each cam cycle, preferably of two minutes, and theother, T2, closes for a short interval immediately following there-closure of contact T1; the cams are mounted on the saine shaft,driven by a motor, not shown, which may be started by applying ground toconductor TST.

At the moment relay M operates, the circuit to relay B is closed at makecontact M1 to ground at break contact E3. At some instant during theensuing two minutes (the timer having been started, or continued inoperation, by the closure of contact D3), Contact T2 closes andcompletes the circuit to relay E, via contact M6. E operates, locks outat make-before-break contacts E1 and transfers the circuit to relay B tocontact T1, now closed, at contacts E3. After a further interval ofapproximately two minutes, contact T1 opens, allowing relay B, if nototherwise held operated, to restore and the ringing circuit releases, aswill be explained. Forced release thus occurs between two and fourminutes after the start of ringing if the latter is not previouslytripped.

lf the called party lifts his handset to answer the call within thisinterval, as is normally the case, a metallic loop is closed across theparty line at the called station, in known manner, and direct currentows from ground at contact B3, via resistor YD or rectifier MRB orcontact K3, as the case may be, the line loop, one winding of relay F,and either break contact Il and resistor YC, or make contact J1 and theringing generator, to battery. Relay F operates its X contact F1,thereby closing the circuit to its other winding on which it operatesfully, and opens the circuit to relay M at break contact F2. Relay Mrestores, opens the circuits to relay K at contact M9, to relays G and Hat contact M3, to relays D and L at contact M7, to relay B at contactM1, to relay E at contact M6, opens the ringing circuit at make contactsM2 and M5 and reconnects the and trunk conductors to relay A at breakcontacts M2 and MS. Relays K, L, if operated, restore. Relay K opens thecircuit to relay C at contact K2 and this relay, if operated, restores.Relay D restores and opens the circuit to relay I at contact D4 and thisrelay, if operated, restores. Relay A re-operates over the loop at thecalled station and re-closes the circuit to relay B, which accordinglyremains operated.

The calling party, upon observing that ringing has ceased, also liftshis handset and can now converse with the called party, transmitter feedbeing furnished to both stations through the windings of relay A.

At the end of conversation, when both parties have replaced theirhandsets, the circuit to relay A is opened and this relay restores,followed by relays B and F in turn. Circuits are closed to the releasemagnets ZMA and 2MB of switches SMA and SMB, respectively, viaoff-normal contacts CMAZ and CMBZ and break contacts B1 and A1, andthese switches restore their wipers to normal, whereupon contacts CMAZand CMB2 open and magnets ZMA and ZMB restore; the ringing circuit isnow in the normal condition. When contact B2 opens, ground is removedfrom the control conductor P of the switch train, releasing the lineiinder and group selector, Fig. l, and the relays CO and LO in the linecircuit of the party line, in the well known manner.

The multi-party line shown in Fig. l is provided with a line circuitarranged to lock out the line in the event of a permanent loop conditionthereon; the line relay L has two balanced windings, connected to groundand battery respectively, thus adapting the line circuit to furnishtransmitter feed to two stations on the line engaged in a revertingcall. In these circumstances, it is clearly unnecessary to retain thereverting call ringing circuit and the intervening switches in theconnection, once the ringing operation has been terminated. However, inthe general case, there will also be party lines connected to the banksof the line finders which are provided with line circuits not arrangedwith lock-out facilities; when a reverting call is made on such a line,it is of course necessary to maintain the connection to the revertingcall ringing circuit throughout conversation. In the general case,therefore, the ringing circuit operates in the manner described,whatever the type of party line circuit involved.

If all the party lines connected to the banks ofthe line finders areprovided with line circuits with lockout facilities, the revertive callringing circuit can be arranged to cause its own release and that of theswitch train at the moment the ringing is tripped; Fig. 5 shows amodification of the circuit of Fig. 2 whereby this may be achieved. Anadditional break contact F3 is inserted between contact B2 and ground.When relay F operates, as described, to trip the ringing, ground isremoved from the control conductor P, releasing the line finder andgroup selector and allowing the cut-ott relay CO of the party line torestore. The line circuit is thereby placed in the lock-out condition,with relays L and LO operated in the well-known manner, which need notbe further described herein, and conversation between the parties mayproceed` Upon the release of the switch train, relay A, Fig. 2, restoresand the ringing circuit is estored to normal as described in thepreceding paragaph.

It will be appreciated by those versed in the art that the invention isnot limited to the precise arrangements which have been described, butthat modifications may be made coming within the scope of the appendedclaims.

What is claimed is:

l. In an automatic telephone system, party lines arranged for dividedringing, party lines arranged for bridged ringing, stations on saidparty lines, a reverting call ringing circuit, selector switches forsetting up a connection from a rst station on any one of said partylines to said reverting call ringing circuit when a reverting call ismade from said first station to another station on said one party line,and means in said reverting call ringing circuit responsive to a ringingsignal transmitted over said connection but only responsive when saidline is arranged for bridged ringing for determining whether said partyline is arranged for divided or for bridged ringing.

2. ln an automatic telephone system, party lines arranged for dividedringing, party lines arranged for bridged ringing, stations on saidparty lines, a reverting call ringing circuit, selector switches forsetting up a connection from a iirst station on any one of said partylines to said reverting call ringing circuit when a reverting call ismade from said first station to another station on said one party line,a source of ringing current, means in said reverting call ringingcircuit for closing a path for ringing current from said source to saidparty line to ring at least one of the stations thereon, anddiscriminating means included in a portion of said path for determiningwhether said party line is arranged for divided or for bridged ringing,said portion of said path being used only when said party line isarranged for bridged ringing.

3. In an automatic telephone system, party lines arranged for dividedringing, party lines arranged for bridged ringing, stations on saidparty lines, a reverting call ringing circuit, selector switches forsetting up a connection from a rst station on any one of said partylines to said reverting call ringing circuit when a reverting call ismade from said rst station to another station on said one party line, asource of ringing current having a rst pole and a second pole, means insaid reverting. call ringing circuit for closing a path for ringingcurrent from said iirst pole'to one side of said party line to ring atleast Vonev of the stations thereon, a rst return path for said ringingcurrent from ground to said second pole and a second return path forsaid ringing current from the other side of said party line to saidsecond pole, and discriminating means included in said second path fordetermining whether said party line is arranged for divided or forbridged ringing.

in an automatic telephone system, party lines arranged for dividedringing, party lines for bridged ringing, stations on said party lines,a reverting Vcall ringing circuit, selector switches for setting up aconnection from a rst station on any one of Vsaid 4party lines to saidreverting call ringing circuit when a reverting call is made from saidfirst static-n to a second station on said one party line, a relay insaid reverting call ringing circuit for determining whether said partyline is arranged for divided or for bridged ringing, a plurality ofsources of ringing current of differing frequencies, contacting deviceseach for interrupting said ringing current according to a different`ringing code, and means, responsive jointly to impulses dialed from saidiirst station and to said relay, for applying ringing current from oneof said sources of the proper frequency to the proper-side of said partyline to ring said second station in the case that said party line isarranged for divided ringing, and for applying to said party lineringing current from one of said sources of the proper requency andinterrupted according to the proper ringing code to ring said secondstation in the case that said party line is arranged for bridgedringing.

5. A telephone system according to claim 4, in which said means forapplying ringing current comprises a multi-position switch having aiirst and a second wiper and contact bank, means for positioning saidswitch in accordance with said dialed impulses, a connection from eachcontact of said rst contact banif` to one of said sources, a switchingrelay connected to selected ones of thercontacts of said second contactbank, and a connection including a contact of said switching relay fromsaid rst wiper to each side of said party line.

6. A telephone system Vaccording to claim 5, in which said means forapplying ringing current further cornprises a connecting relay having acontact included in eachV of said last-mentioned connections, and aconnection from said connecting relay including a contact of saidswitching relay to each of said contacting devices.

7. in an automatic telephone system, party lines arranged for dividedringing, party lines arranged for bridged ringing, stations on saidparty lines, a reverting call ringing circuit, selector switches forsetting up a connection from a first station on any one of said partyiines to said reverting call ringing circuit when a reverting call ismade from'said iirst station to a second station on said one party line,a relay in said reverting call ringing circuit for determining whethersaid party line is arranged for divided or for bridged ringing, aplurality of sources of ringing current of differing frequencies,contacting devices each for interrupting said ringing current ,12according to a different ringing code, and means responsive'j'ointly toimpulses dialed from said tirst station and to said relay, for applyingringing current fromone of said sources of therproper frequency to theproper side of said party line toring said second stationin the casethat said party line is arranged for divided ringing, said ringingcurrent being interrupted invariably according to a particular one ofsaid ringing codes, and for applying toV said party line ringing currentfrom one of said sources of the proper frequency and interruptedaccordto the proper ringing code to ring said second station in the casethat said party line is arranged for bridged ringing. f

8. in an automatic teiephone system, party lines arranged for dividedringing, party lines arranged for bridged ringing, stations on saidparty lines, a reverting sail ringing circuit, selector switches forsetting up a connection from a rirst station on any one of said partylines to said reverting call ringing circuit when a reverting cail ismade from said first station to a second station on said one party line,a relay in said reverting call ringing circuit for determining whethersaid party line is arranged for divided or for bridged ringing, apiurality of sources of ringing current of differing frequencies,contacting devices each for interrupting Vsaid ringing current accordingto a different ringing code, and means responsive jointly to impulsesdialed from said iirst station and to said relay, for applying ringingcurrents from said sources of the proper frequencies to the proper sidesof said party line to ring said tirst and second stations alternately inthe case that said party line is arranged for divided ringing, and forapplying tosaid party line ringing currents from said sources or" theproper frequencies and interrupted according to the proper ringing codesto ring said tirst and second stations alternateiy in the case that saidparty line is arranged for bridged ringing.

9. A telephone system according to ciairn 8, in which said revertingcali ringing circuit further comprises discriminating means, etiectiveonly when said party line is arranged for bridged ringing, forpreventing the application to said party line of ringing currentinterrupted according to the proper ringing code to ring said iirststation when one and the same ringing current frequency is the properfrequency to ring both said rst and said second stations.

10. A telephone system according to ciaini 8, in which said revertingcall ringing circuit further comprises timing means for causing theseverance of said connection and the release of said reverting callringing circuit if said reverting call is neither answered at saidsecond station nor cancelled from said first station within apredetermined time interval.

References Cited in the file of this patent UNITED STATES PATENTS1,487,996 Williams Mar. 25, 1924 2,411,525 Deakin Nov. 26, 19462,628,281 Morris Feb. 10, 1953

